Chromates, including sodium chromate or chromic acid, as well as many other compositions containing the chromium moiety are often referred to generically as "chrome". Chrome-containing compositions enjoy widespread industrial usage but their residues are objectionable from an environmental standpoint to the extent that they form a constituent of the waste water effluent. The use of chrome, especially as chromates, is, however, essential in many industrial operations. For example, their use is very important as an after-rinse for metals which have been subjected to a so-called "conversion coating" for the purpose of enhancing the metal's corrosion resistance and paint bonding characteristics. Examples of conversion coating processes may be found in U.S. Pat. Nos. 2,819,193, 2,928,762 and 2,936,254. An example of a chrome after-rinse process suitable for use in conjunction with a conversion coating process may be found in U.S. Pat. No. 3,863,877. Spent chrome rinse baths and wash water rinse solutions contain small (usually less than 100 ppm) amounts of the chromate CrO.sub.4.sup.-- or dichromate Cr.sub.2 O.sub.7.sup.-- ions. Chrome as part of these ions appears with a valence of +6, in which condition it is both toxic and soluble. To remove the chrome, which in the +6 valance state is known as chromyl, it is necessary to reduce the valence state to +3, referred to as chromic. In the lower valence state chromic hydroxide, Cr(OH).sub.3, may then be precipitated. The reduction of the chrome ion from +6 to +3 may be accomplished satisfactorily at a low pH of below about 5.0. The reducing agents for this purpose are well known and include metallic iron, ferrous sulfate, sulfur dioxide and the potassium or sodium sulfites, bisulfites or metabisulfites. Following reduction of the chrome ion to its +3 valent state, caustic soda or lime has been disclosed as suitable to precipitate chromic hydroxide.
The removal of chrome from chrome-containing waste water by precipitation of chromic hydroxide is known in the art. By way of illustration, copending U.S. application Ser. No. 917,506 discloses a process for the purification of chrome-containing waste water by treating the waste water with both a reducing agent suitable for converting hexavalent chrome to trivalent chrome and a magnesium, calcium, ferric or aluminum sulfate or chloride salt. Following the reduction step, the pH of the now acidic solution is raised with caustic to cause the formation of a chromic hydroxide precipitate. As a further example, Japanese Pat. No. 47-17697, issued Sept. 9, 1972, discloses precipitating chromic hydroxide from waste water by the process of (a) reducing hexavalent chromium in the waste water to trivalent chromium at a pH of about 2 using a reducing agent; (b) neutralizing with alkali at a pH of at least 8; and (c) adding magnesium ions (either prior to or after the reduction and neutralization steps) to precipitate the chromic hydroxide. As another example, Japanese Pat. No. 71-16389, published May 6, 1971, discloses a process for treating chromium containing waste water comprising reducing hexavalent chromium to trivalent chromium and adding to the resulting solution of reduced chromium a mixture of aluminum sulfate and sodium hydroxide or calcium hydroxide in order to effect precipitation of chromic hydroxide.
Special problems are associated with chrome-containing waste water solutions which also contain a chelating agent for trivalent chromium such as, for example, ethylenediaminetetraacetic acid. Once the hexavalent chromium is neutralized to trivalent chromium, the chelating agent will tend to complex the trivalent chromium, thereby making removal of the chromium from solution difficult. It has now been found that ferric and aluminum (added as ferric or aluminum chloride or sulfate) are surprisingly effective in producing the precipitation of chromic hydroxide from waste water despite the presence of a chelating agent for trivalent chromium.